The field of the invention is that of telecommunications and more particularly that of Internet Protocol/Multi Protocol Label Switching (IP/MPLS) networks.
The MPLS technology is not described here; for more information on this technology see the Internet Engineering Task Force (IETF) document Request for Comments (RFC) 3031.
The context of the invention is that of IP/MPLS networks including core routers, referred to as P routers, and access routers also referred to border routers (ASBR).
Some IP/MPLS networks, referred to as Flat IP networks, use IP switching in all core routers situated between the border routers.
Other IP/MPLS networks use MPLS switching in all core routers situated between the border routers, the border routers using MPLS tunnels to send IP packets. In this technique, which is more particularly described in section 4.6 “LSP Tunneling between BGP Border Routers” of the above-mentioned document RFC 3031, the core routers do not need to know the external routes announced by the border routers using the Border Gateway Protocol (BGP).
In pure BGP terminology, an ASBR is a border router of an autonomous system setting up an eBGP session with another autonomous system. The present document uses the concept of an output router of an autonomous system that corresponds to a more extensive definition of an ASBR and that, by definition, applies to any border router of the autonomous system announcing BGP routes for which it is an obligatory passage point, which means either that it is the BGP next hop or that the BGP next hop belongs to one of its interfaces.
Some communications services have a high demand in terms of availability (for example voice (VoIP) services and telemedicine services). These services require deterministic rerouting within less than 100 milliseconds (ms) in the event of a failure affecting the link or node. At present, the only technology offering such rerouting performance is the Fast Reroute technology that sets up in advance local back-up paths bypassing the protected element. In the event of a failure, the directly upstream node updates its routing table and diverts the traffic to the back-up path. This method requires no route calculation or signaling after the failure has occurred. Moreover, the back-up routes are pre-installed in the routing tables of the routers, which guarantees a deterministic rerouting time of less than 100 ms.
There are two modes of providing this protection:                the MPLS Fast Reroute mode, based on setting up end-to-end MPLS-TE primary tunnels locally protected by MPLS-TE back-up tunnels, this mode being described in the IETF document RFC 4090;        the IP Fast Reroute mode, based on protection of IP routes by back-up routes avoiding the protecting element and with no risk of loop; these back-up routes can be in connected mode with local back-up MPLS-TE tunnels or in non-connected mode if there is no risk of loop; for more details of this second mode see, for example:            Shen, Pan, “Nexthop Fast ReRoute for IP and MPLS” (http://www.potaroo.net/ietf/all-ids/draft-shen-nhop-fastreroute-01.txt); and    Shand, Bryant, “IP Fast Reroute Framework”, http:/www.ietf.org/internet-drafts/draft-ietf-rtgwg-ipfrr-framework-05.txt.
The MPLS Fast Reroute mode, with meshing of the border routers by primary MPLS-TE tunnels, protects ASBR-P, P-P links and P nodes. It is not appropriate for large-scale networks because it requires meshing of all the ASBR and therefore requires a number of tunnels proportional to the square of the number of ASBR. It is therefore in practice applicable only to a small number of ASBR (approximately 100).
The IP Fast Reroute mode protects ASBR-P, P-P links and P nodes. It requires no MPLS-TE primary tunnels and is therefore more appropriate for large-scale networks.
Present day Fast Rerouter techniques as described above cannot protect the ASBR of an IP/MPLS network.
This is because the MPLS Fast Rerouter technique, as defined in the document RFC 4090, which requires RSVP-TE tunnels between the autonomous sites, cannot be used in large-scale networks.
Moreover, the IP Fast Reroute technique does not protect the ASBR, which are situated at the ends of tunnels or consist of designated output points of the network and can therefore not be bypassed in the event of a failure.
Moreover, present-day protection techniques use only the information of the Internal Gateway Protocol (IGP), which is an internal routing protocol, and can provide back up only for failures affecting internal elements of the network. Thus they cannot be used to back up the ASBR (border routers between two autonomous sites), protection thereof making it necessary to take account of external routing information distributed using the BGP protocol.
The only mechanisms providing back up against ASBR failures available at present are based on convergence of the BGP and provide back-up times exceeding one second, which is incompatible with the availability demands of real-time services.
To ensure good client router to client router availability, it is therefore essential to define new mechanisms providing fast ASBR protection and capable of supporting a large number of client routers (i.e. suitable for scaling up).